Method of Maintaining Separation Between Vehicles in a Fixed Guideway Transportation System Using Dynamic Block Control

ABSTRACT

The present invention relates generally to ground transportation systems, and more particularly to a fixed guideway transportation system that achieves a superior ratio of benefits per cost, is lower in net present cost and thus more easily justified for lower density corridors, and can provide passenger carrying capacities appropriate for higher density corridors serviced by mass rapid transit systems today.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 13/218,429 filed Aug. 25, 2011, which claims priority to U.S. Provisional Application No. 61/459,247, filed Dec. 10, 2010, the contents of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to ground transportation, and more particularly to fixed guideway transportation systems having an optimal ratio of benefits per unit cost and a method for designing the same.

BACKGROUND OF THE INVENTION

Modem mass rapid transit rail systems are very effective carriers of people. They are generally grade separated systems to enable vehicles to operate unaffected by automobile traffic, and thereby are able to achieve traffic densities otherwise unachievable. They are, however, very expensive. A typical, but conservative order of magnitude system capital cost for a system is approximately $100 million per bi-directional track mile of system, making it difficult for communities and cities to justify and/or afford the cost of new construction. This limitation has the effect of constraining the reach of these systems, and thus limiting the convenience to the users who can only ride the systems to the few locations to which guideway has been constructed. This results in a classic case of Catch 22. The high cost of systems requires a high ridership to justify the cost. However, high guideway costs limit construction and thus the reach of fixed guideway systems. This limits convenience to the riders, making it difficult to achieve the high ridership needed to justify the high cost.

Conventional mass rapid transit rail technology attempts to improve the ratio of benefits per unit cost by focusing on serving the commuting public. This means building systems to achieve very high passenger capacities to major employment centers. An example conventional system is shown in FIG. 1. As shown, conventional systems 110 achieve high capacities by building heavy infrastructure and operating long heavy trains 112 that typically carry a large number of riders to the few large employment centers 114, 116 that they can most effectively service, while bypassing smaller towns or communities 118, 120. This, however, requires very costly guideway 122 and station structures 124, 126, which limits the system's reach and thus convenience for the users, especially for those who want to travel to the generally more widely distributed retail, residential, or recreational destinations.

With guideway 122 and station structures 124, 126 that must be built to handle long heavy trains 112 to support demand during commute hours, the result is an expensive but marginally justifiable solution for commute hour travel which is far too expensive to justify for other periods of the day and other destinations.

Other existing transportation systems that aim to be less expensive to build and operate include automated people mover (APM) systems, such as those operating in many modem airports and some cities. These systems are low speed/low capacity systems that operate driverless vehicles at speeds in the range of 25 to 30 mph and achieve line capacities in the range of 2,000 to 3,000 passengers per hour per direction. Given the limited speed and capacity of these systems, even with the somewhat lower cost of construction due to the use of smaller vehicles, the benefit per cost is still poor. Furthermore, with the lower speeds and line capacities, these systems are limited in utility to local service routes.

Another type of transportation system that has been discussed is called “personal rapid transit” (PRT). PRT's differ from the more common APM systems in that these systems are built with offline stations which allow higher traffic densities to be achieved. Typically these systems operate driverless cars that seat four to six people and can provide service on a personal demand-driven basis. However, with the very small cars, high speeds are difficult to achieve and line capacities are severely restricted. There is one PRT that is operating at West Virginia University, the Morgantown PRT, which is an 8.2 mile long system having cars that seat 20 people. With a claim of 15 second headways, a line capacity of 4,800 passengers per hour per direction can be achieved. With rubber-tired vehicles, however, the top speed of the system is 30 mph thus limiting its applicability to low speed local service lines.

Co-pending U.S. application Ser. No. 13/218,422 (CTI-001), the contents of which are incorporated by reference in their entirety, dramatically advanced the state of the art by providing a fixed guideway transportation system that can overcome many of the above and other challenges of the prior art. For example, the system of the co-pending application includes driverless vehicles carrying 10 to 30 persons designed for optimal ratio of benefits per cost. However, certain challenges remain.

For example, in order to cost effectively build and operate a system that operates smaller vehicles such as those contemplated by the co-pending application, yet achieves line capacities that justify the cost of constructing track infrastructures, the density of traffic that can be achieved needs to be sufficiently high. That means that safe operating headways must be made smaller than those achievable with conventional control systems that represent today's state of the art. Furthermore, these safe operating headways should be achieved at mass rapid transit speeds (at least 60 mph). This cannot be achieved with current systems. Accordingly, there remains a need for a methodology for designing a system that provides headways necessary to operate at these high traffic densities.

Relatedly, since a collision between two vehicles is a life-threatening event, control functions that prevent collisions are critical to safety. In the rail industry, control that is critical to safety must be designed and implemented to a standard commonly referred to as “vital.” In recent years achieving vital status has required an analytical demonstration of a Mean Time Between Unsafe Event or Hazard (MTBH) of 10⁹ hours or greater. Accordingly, any methodology aimed at increasing traffic density should include collision protection satisfying this standard.

SUMMARY OF THE INVENTION

The present invention relates generally to ground transportation systems, and more particularly to a fixed guideway transportation system that achieves a superior benefit per cost ratio, is lower in net present cost and thus more easily justified for lower density corridors, and can provide passenger carrying capacities appropriate for higher density corridors serviced by mass rapid transit systems today.

According to certain aspects, the present invention provides a methodology for preventing collisions between vehicles that limits the rise in headway as the vehicle speed increases. Thus the invention allows transportation systems to achieve shorter time separations between vehicles traveling at high speeds thus significantly improving the utility of fixed guideway infrastructure.

In accordance with these and other aspects, a method of controlling a plurality of driverless vehicles in a fixed guideway system according to the invention includes determining that there are no fixed obstacles in the system, and maintaining separation between the vehicles using dynamic block control that takes into account a controlled emergency braking rate for all of the vehicles in the system.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein:

FIG. 1 illustrates a conventional mass transit system;

FIG. 2 illustrates how a conventional “brick-wall criteria” is used to determine achievable headway;

FIG. 3 is a chart illustrating a relationship between headway and speed in an example rail system;

FIG. 4 is a flowchart illustrating a general collision prevention methodology according to embodiments of the invention;

FIGS. 5 and 6 illustrate aspects of Worst Case Stopping Distance that is used to maintain safe separation between vehicles in example systems;

FIG. 7 illustrate a safe separation between vehicles that is maintained according to aspects of the invention;

FIG. 8 is a diagram illustrating an example transportation system implementing a collision prevention methodology according to embodiments of the invention;

FIG. 9 is a functional block diagram illustrating one example of how control functionality is segregated between physical components in embodiments of the invention;

FIG. 10 is a hierarchical block diagram illustrating an example control system implementing a control system methodology according to embodiments of the invention; and

FIG. 11 is a block diagram further illustrating one example of a control system implementing a control methodology according to embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference to the drawings, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and examples below are not meant to limit the scope of the present invention to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. Embodiments described as being implemented in software should not be limited thereto, but can include embodiments implemented in hardware, or combinations of software and hardware, and vice-versa, as will be apparent to those skilled in the art, unless otherwise specified herein. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the invention is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present invention encompasses present and future known equivalents to the known components referred to herein by way of illustration.

According to certain aspects, the invention of the co-pending application enables the construction of rail lines that: 1. achieve a superior amount of benefits per cost; 2. are lower in cost and thus more easily justified for lower density corridors; and 3. can provide passenger carrying capacities appropriate for higher density corridors serviced by mass rapid transit systems today.

In certain embodiments, these objectives are met by utilizing smaller vehicles that can operate on a less expensive infrastructure. Using certain methods according to the co-pending application, the costs of fixed guideway mass rapid transit systems are reduced, allowing more destinations to be accessed. Also, with certain methods according to the co-pending application, the same structures appropriate for low ridership corridors and/or service hours can be used to achieve passenger carrying capacities needed for the high capacity corridors served today by modern mass rapid transit systems.

According to further aspects, the invention of the co-pending application improves the amount of benefits per cost of rail transit by reducing the cost to levels more justifiable for low density corridors. To be meaningful, certain methods according to the co-pending application achieve improved benefits per cost in a holistic manner, in other words, by reducing the net cost of ownership which includes not only the cost of equipment but also the net cost of operating and maintaining the system.

Although the principles of the inventions of the co-pending application and the present application will be explained in connection with applications to conventional diesel and/or electrified rail systems, the invention is not limited to these types of systems. For example, the principles of the invention can be extended to conventional and other vehicle technologies that do not rely on steel wheels rolling on steel rail.

According to certain aspects, the present inventors recognize that increasing traffic density, such as that contemplated in the system according to the co-pending application, cannot be achieved with conventional collision avoidance and vehicle control methodologies.

A novel vehicle control methodology that can be used in a system according to the co-pending application, as well as together with the present invention, is described in U.S. application Ser. No. 13/323,768 (CTI-008), the contents of which are incorporated herein by reference in their entirety.

Accordingly, the present application is directed to novel collision avoidance methodologies for use in a fixed guideway transportation system such as that described in the co-pending application, and which improves traffic density. For example, the present inventors recognize that the traffic density that is achievable with conventional vehicle control systems is limited by what is commonly referred to as the “brick wall criteria,” a control rule that mandates that a vehicle following another vehicle must follow at a distance such that collisions are avoided even if the location of the tail end of the leading car were a brick wall on the track. This, coupled with the low levels of adhesion and therefore low deceleration rates (typically −1.5 mphps to −2.5 mphps) that can be guaranteed with steel wheels and steel rail, prevents vehicles from operating at time separations (headways) that achieve service capacities that can justify the high cost of building track infrastructure.

Furthermore, the impact of the “brick wall criteria” on capacity is non-linear with the impact increasing rapidly at-speeds that exceed about 15 mph. The reason for this is shown in FIG. 2. Headway is calculated by dividing the Headway Distance by the speed of the following vehicle and line capacity (vehicles per hour) is calculated by dividing the Headway into 3600 (the number of seconds in an hour). Since the Headway Distance is the sum of the Separation Following Distance and the length of the leading vehicle X_(length) 208, at low speeds, the vehicle length, which is of course a fixed length, can be large relative to the Separation Following Distance 206 and dominates the Headway Distance. Thus as the speed increases (in the range of from 0 to about 6 mph), the time to travel the Headway Distance becomes shorter. However, as the speed continues to increase above about 6 mph, the Separation Following Distance, which increases with the square of the velocity of the following care, becomes the predominant component of the Headway Distance. The headway at these increased speeds, which is calculated by dividing the headway distance by the velocity of the trailing vehicle, thus increases roughly linearly with velocity. An illustration of this relationship between headway and speed is shown in FIG. 3 which plots achievable headways as a function of vehicle velocity assuming a braking rate of 1.5 mphps. Note the headway dropping with speed at the low speeds but rising nearly linearly at the higher speeds.

The push to increase the utility of fixed guideway infrastructure by achieving shorter headways has a long history going back to the early days of rail in the U.S. However, since the present invention advances the state of automatic train control, it is useful to note the more recent evolution of technologies applied to the control of driverless vehicles.

The traditional approach to collision avoidance has been to assume the use of failsafe emergency braking as the singular, “always safe,” response to detected unsafe situations. Emergency braking equipment is designed to achieve a brake rate typically referred to as the guaranteed minimum achievable braking rate which is selected in large measure by the predicted available adhesion between the wheel and the rolling surface. This braking rate is determined by collecting large volumes of field data on the system to be controlled and the control system is then designed to be failsafe with vehicle protection logic that makes the assumption that the trailing vehicle will always brake at a rate that is greater than the guaranteed minimal achievable braking rate when emergency braking is called upon. No assumption is made, however, as to the guaranteed maximum achievable braking rate of the leading vehicle, meaning that the logic assumes an upper braking rate of infinity for the leading car. Thus the basic premise of all past control systems is that the lead car can brake to a stop instantaneously.

The focus of other attempts to reduce the achievable headways of vehicular control systems can be separated into three categories. They are 1) improve vehicle tracking accuracy, 2) increase braking rate, and 3) improve reaction time. A brief discussion of each follows.

Improve tracking accuracy: The performance of a control system is dependent to a large degree on the accuracy to which vehicles can be tracked. Since uncertainty in the location of the vehicle needs to be added as a safety margin to the vehicle safe separation distance, tracking uncertainty can have a serious impact on the performance of a system.

First, until recent advances in communication and control technology, vehicular control systems were designed on a vehicle tracking technique referred to as “fixed block.” Fixed block referred to an approach whereby the track on which vehicles operated were segregated into blocks, fixed in place and defined by physical sensors in the trackway. The tracking resolution was therefore limited to the distance between the installed physical sensors and was generally quite poor. Improvement of tracking resolution, and therefore system performance, required the installation of more sensors to make each block smaller. The drawback was that as the number of sensors increased, reliability was impacted as there would now be more components that can fail and must be maintained.

A next form of tracking is called communication based train control (CBTC). With CBTC, tracking resolution was improved by having the vehicle track its movement in reference to fixed markers in the trackway and communicate its position information to the controlling computer via communication technology between the vehicle and the station. This allowed vehicles to be tracked with a resolution finer than that defined by the placement of sensors in the trackway. Early designs relied on close-proximity inductive communication loops that served as a medium for coupling frequency shift keyed data between vehicle and station and vice versa. More recent systems have used radio frequency (RF) communication technology to communicate encrypted data using radio frequencies. One tracking technology pioneered by BART measures the propagation time of radio signals to measure the location of trains relative to fixed ground based radios.

Another approach of tracking attempted in the past uses forward looking radar and/or Laser Detection and Ranging (LIDAR) devices on vehicles to detect the presence of obstacle vehicles. This approach may work well for preventing collisions between vehicles on the same track but does not work well at system merge points where it becomes impossible to detect the presence of other vehicles on a merging track.

Increasing the braking rate: Headways between vehicles can be shortened by increasing the braking effort achieved by the vehicles. Various approaches have been used to achieve the higher brake rate but ultimately the braking rate is limited by what the human body can tolerate.

One system being developed in Japan in the 1970's called the Computer Controlled Vehicle System (CVS) used a braking system that could achieve a 0.5 G deceleration force by relying on an explosive device under the car that would cause the braking system to grab the rail during emergency scenarios.

Another braking approach used by some conventional systems today, including San Francisco Muni, relies on the use of skid brakes. These systems use skid brakes for auxiliary braking to achieve a rapid deceleration that is greater than what can be achieved by the wheel to rail interface. With skid brakes, a skid plate is dropped onto the ground to augment the braking effort otherwise available from the wheel to rail interface when emergency braking is required to achieve a safe high rate emergency stop.

Improving the reaction time: To a limited degree headways can be shortened by improving the reaction time of the control system. One program sponsored by the Urban Mass Transit Administration (UMTA) called the Advanced Group Rapid Transit (AGRT) System R&D Program reduced reaction times to 40 ms to achieve the goal of three second headways between vehicles. The three seconds, however, was only achievable at about 15 mph.

According to other aspects of the invention, the present inventors recognize that the traffic density that can be achieved on any given line of rail service while maintaining collision prevention is affected by the rate at which vehicles can be made to pass the most headway restricted point on the line. Since safe operation requires that vehicles must always be able to stop before arriving at obstacles on the track, with all track geometries (i.e. grade, track curvature) being equal, the greatest restriction will occur where there are fixed obstacles (i.e. zero speed obstacles) in the path of the vehicle.

Therefore, in order to achieve high traffic densities, a system is preferably implemented in two steps, as shown in FIG. 4. First all zero speed obstacles are eliminated from the path of vehicles that operate on the tracks (step S402). Second, a vehicle control system is developed and deployed that can take into consideration the dynamic state of the obstacle vehicles when determining the safe separation that can be allowed between two vehicles traveling on the same track (step S404).

With regard to the first step S402, there are two types of zero speed (i.e. fixed) obstacles that can exist in a system. First are the obstacles that are physically fixed to the infrastructure and thus are always present (i.e. fixed location obstacles). The second are vehicles on the track that must be treated as zero speed obstacles when they are stopped but not necessarily so when they are moving.

In conventional systems there are three different types of fixed location obstacles. First, for conventional systems, the vehicles stopped at station platforms are stationary obstacles to other vehicles approaching the platform. This means that vehicles at platforms, even if there were no other obstacles in the system become the capacity limiting feature on the line since the line capacity is limited by the worst location on the line.

Second, on systems that use control technology that are based on fixed block technology, by the very fact that detection blocks are fixed in space, requires that the leading edge of the occupied block in front of a trailing vehicle must be treated as fixed obstacle locations for the following vehicle.

Third, on conventional rail, the points of switch are fixed obstacles on the track. This is because while a switch is moving and is in an intermediate position, a vehicle arriving at the switch before it has locked itself in the new position will derail.

The present inventors recognize that the need to accommodate each of the three fixed location obstacles described above limits the capacity of a rail service line. Thus, until a means is implemented to eliminate these constraints, there is little advantage to changing the control rules to account for moving vehicle obstacles.

A collision prevention methodology that allows one to overcome the restriction imposed by zero speed obstacles thus preferably eliminates all three of the fixed location obstacles described above. The present inventors' recognition of the summed effect of eliminating all three types of obstacles as being unexpectedly beneficial to increasing traffic density on a rail line is one aspect of the invention.

First, to eliminate the station platform obstacles, there are two different situations that are preferably addressed, each requiring different approaches. First, stations that are needed at locations that are not at the ends of the system, referred to in this application as mid-line stations. Second, stations at the ends of lines, or end-of-line stations where vehicles must stop and then turn back to travel in the direction from which it came. The present inventors recognize many different ways of eliminating the stopped vehicle as an obstacle for both such situations. Example approaches are described in co-pending U.S. application Ser. No. 13/218,422 (CTI-001).

Next, the present invention addresses the need for eliminating the constraints of Fixed Block control systems. The present inventors recognize that Fixed Block systems have control inefficiencies due to the poor location resolution that derives from the long detection blocks that are characteristic of these systems. In other words, as shown in FIG. 5, these systems segregate a track into detection blocks 502-A, 502-B, 502-C, etc. In one existing system, the SF Bay Area Rapid Transit System (BART), the length of these blocks range from about 100 to 1200 feet. If a train 504 occupies block 502-C, the train 506 traveling in a direction toward block 502-C must assume there is a brick wall 508 at the closest edge of block 502-C, regardless of where train 504 actually is. A worst case stopping distance 510 is calculated from this brick wall 508.

To address the inefficiencies inherent with Fixed Block controls, embodiments of the invention utilize Communication Based Train Control (CBTC) systems that rely on rapid and reliable communication of vehicle position data from the vehicle to the control computers in the station. As shown in FIG. 6, with CBTC systems, the vehicle position data is commonly (but not necessarily always) determined by the vehicle 604 using markers installed in the trackway (not shown) as position references and determining longitudinal position along the track by measuring the distance traveled between markers using tachometers or other longitudinal movement measuring devices. This information is communicated by vehicle 604 to a station and the position of the vehicle 604 is determined to be the location of the reference marker in the guideway plus the distance traveled since the last marker was detected. If the traveled distance is updated frequently, this provides the station with much better position resolution than what is provided by Fixed Block control systems.

Accordingly, in conventional systems in use and in development today, the tail end of the leading car 604 is assumed to be the location of a brick wall 608, but one that is moving with the movement of the vehicle 604 as is illustrated conceptually in FIG. 6. Thus, currently developed CBTC systems are also referred to as moving block system.

In embodiments of the invention, meanwhile, the tail end of the leading train 604 will still be an obstacle to the following train 606 but, since information is available as to the current dynamic state (velocity and perhaps even acceleration and jerk), this information will be used in the determination of what will be considered a safe following distance 610 for the trailing car 606. Thus the method described in connection with embodiments of the present invention will be referred to as a dynamic block control, which is a new term in the art, and should not be confused with any existing control methodology.

Although the concept of Communication Based Train Control is not by itself an aspect of the invention, its use is beneficial to achieving performance otherwise made possible by the invention. Note, the typical times between vehicles that can be achieved with conventional CBTC approaches is about 50 seconds at 60 mph. One possible way in which CBTC technology is used in the present invention to achieve even higher traffic densities than what is achievable with conventional moving block systems is described in more detail in U.S. application Ser. No. 13/316,402 (now U.S. Pat. No. 8,725,325) (CTI-007), the contents of which are incorporated herein by reference.

Finally, the present invention addresses the need to eliminate the fixed obstacles imposed by movable switches in the rail. In conventional systems, the rail at the point where the track diverges must be moved using mechanical machines that effectuate the move. Since the rail is typically a very heavy piece of steel, this movement takes time (a few seconds). Furthermore, since it is unsafe to move a train over the point of switch if the movable rail is not locked in place, time must be allowed for the switch to lock, and then for it to report to the station that the locking mechanism has been engaged before, the control logic can allow movement of a vehicle over the switch. As a result, after one vehicle passes a point of switch, if the next vehicle is to take a different route over the switch, an immovable obstacle must be assumed to exist at the switch point until the first vehicle passes and the switch has moved and locked in the new position.

To eliminate this switch point obstacle, embodiments of the invention select and enforce the route through without the movement of the rail in the guideway. In other words, the path that the vehicle will take will be enforced by equipment on board the vehicle instead of in the rail. This allows the direction of travel to be selected well before the vehicle arrives at the point of switch, thus allowing the control logic to ignore the point of track divergence as an obstacle.

In order for a system to safely overcome the restriction imposed by switch point obstacles, there are two methods that are preferably implemented. The first is a method for mechanically selecting and enforcing the direction of travel of a vehicle through a point of switch. The second is a method for integrating the switch mechanism and the vehicle control logic in a way that ensures safe operation at all times. Example implementations of these methods are described in U.S. application Ser. No. 13/323,759 (now U.S. Pat. No. 8,706,328) (CTI-005), the contents of which are incorporated herein by reference in their entirety.

According to further aspects, and as mentioned above, the present inventors recognize that with conventional vehicular control systems, the emergency braking is implemented as failsafe with the safe failure mode being to brake as hard as is physically achievable. The braking force achieved is considered to be a minimum achievable force with no limitations assumed on what is the maximum achievable force. Taken to an extreme this requires that one assumes that an infinite brake force might be achievable at times, which in turn requires the safe vehicle separation distance to be calculated with the assumption that the leading vehicle can stop instantaneously.

In embodiments, a collision prevention methodology according to the invention implements a braking system that is designed to guarantee not only a minimum achievable braking force but one that also guarantees a maximum achievable braking force. This is achieved by deploying an emergency braking system that targets a specific brake rate and achieves the targeted rate +/− control error with a degree of reliability that supports the MTBH safety criteria for the system. The safety monitoring logic can then assume that all vehicles brake at the target rate +/− control error in determining what is or is not a safe following distance between cars. The smaller the difference between the deceleration rates of the leading and following car, the smaller the distance can be between the two cars. With a control error in the range of 3%, a headway less than 9 seconds can be achieved even with the vehicle traveling on a negative slope where worst case maximum stopping distances can be very long.

Example implementations of a braking system that is capable of achieving a target brake rate for use in the present invention are described in U.S. application Ser. No. 13/316,398 (now U.S. Pat. No. 8,744,652) (CTI-006), the contents of which are incorporated herein by reference in their entirety.

Returning to FIG. 4, having eliminated all fixed obstacles in the system and having implemented a braking system that assures a guaranteed controlled brake rate as described above and in the incorporated patent applications, a next step S404 of a method according to the invention implements collision avoidance logic that relies on vehicles braking no more than or no less than the target brake rate plus or minus a small control error during emergency braking

Embodiments of the invention implement a monitoring function that resides wayside (i.e. not on a vehicle) that is updated with new information about the state of the system (i.e. position, speed, and perhaps acceleration of all vehicles and perhaps even condition on the track) on a very frequent basis (e.g. 500 ms). The monitoring function continuously affirms that potentially unsafe situations have not occurred and sends a Safe to Proceed (STP) code to each vehicle that enables the withholding of emergency brakes on each vehicle receiving the code for a short time interval. On the vehicle, receipt of the STP withholds emergency braking for 1100 ms, for example, so as long as a transmitted STP code is not missed two consecutive times, vehicle movement will be allowed. Loss of the STP results in a latched condition on the vehicle causing the braking to be irrevocable until at least the vehicle has come to a complete stop.

There are a variety of system states that are potentially unsafe and are preferably detected by the monitoring function. They are:

Case 1—a vehicle following a car in front too closely

Case 2—two vehicles coming together at a merge on a trajectory that can lead to collision

Case 3—two vehicles traversing diverging points on the track on a trajectory that can lead to collision

Case 4—two vehicles traveling toward each other at a speed from which a stop cannot be achieved before a collision would occur

Case 5—a vehicle traveling at a location and speed that exceeds or will exceed a civil speed limit (including a stop point or closed gate) on the track

In all instances, a Safe Separation Distance must be maintained between vehicles and points of potential collisions. Of the above, the first three scenarios are affected by the use of Fail Operational Braking and thus will be addressed in more detail below.

Were this a conventional control system, the Safe Separation Distance for each of cases 1 through 3 would be the Worst Case Stopping Distance of the trailing vehicle, because the conventional approach assumes that the leading vehicle can come to an immediate stop. In contrast, embodiments of the invention assume the use of a Vital Fail Operational Braking System, meaning that it is assured that all vehicles will brake at a targeted rate plus or minus a control error. With this assumption, the vehicle to vehicle separation distance that must be maintained by this control system can be as illustrated in FIG. 7. As shown in FIG. 7, the Safe Separation Distance between leading vehicle 704 and trailing vehicle 706 is defined by the Stopping Distance Delta 710, which is the difference between the Worst Case Stopping Distance of the following vehicle 706 and the Best Case Stopping Distance of the leading vehicle 704.

Collision prevention or avoidance according to embodiments of the invention thus preferably monitors the system state at all times to confirm that this Safe Separation Distance is never violated. When a violation is detected, the STP is removed from the trailing vehicle (in the case of the merge, the vehicle further away from the merge point) and the vehicle is brought to a stop. Moreover, if equipment failure results in loss of communication to all vehicles, all vehicles will cease to receive the STP and all will brake at the targeted rate plus or minus the control error and collisions will be averted.

A collision prevention methodology that employs a braking distance and separation monitoring and communication scheme as above and can be included in embodiments of the present invention is described in more detail in co-pending U.S. application Ser. No. 13/218,434 (now U.S. Pat. 8,554,397) (CTI-004), the contents of which are incorporated herein by reference in their entirety.

FIG . 8 is a diagram illustrating an example fixed guideway transportation system implementing the collision prevention methodology according to the present invention. As shown, the system 800 includes stations 802 and track 804 implemented with immovable switches 812. Vehicles 808 run on the track 804 and are controlled to operate at a separation distance allowed by the Dynamic Block Control system 806. Safe vehicle separation and thus collision prevention is ensured by the Dynamic Block Control logic resident in the Station Level Controllers which monitors the system state to ensure that if the safe separation distances between cars 808 are ever violated a safe system response will be taken. The station level controllers further exchange Reports and Requests from the Central Traffic Management Center 810 and control vehicles 808 in accordance with the requests received.

Embodiments of the invention implement a fixed guideway transportation system 800 such as that disclosed in, and with vehicles 808 that are sized in accordance with the principles of, co-pending U.S. application Ser. No. 13/218,422 (CTI-001). Moreover, although the principles of the inventions of the co-pending application and the present application are explained in connection with implementations using conventional diesel and/or electrified rail systems, the invention is not limited to these types of systems. For example, the principles of the invention can be extended to other vehicle technologies that do not rely on steel wheels rolling on steel rail.

Although the track 804 shown in FIG. 8 is a simple bi-directional line, this example is not limiting, and track 804 may comprise a more complex route including various interchanges where two or more tracks (bi-directional and/or uni-directional) come together and intersect.

In accordance with the high-density collision prevention principles of the present invention, all fixed obstacles have been eliminated from vehicles 808 running on track 804. Accordingly, stations 802 are off-line, for example using mid-line and/or end-of-line platforms such as those described in co-pending U.S. application Ser. No. 13/218,422 (CTI-001). Moreover, collision prevention system 806 implements communication based train control such as that described in co-pending U.S. application Ser. No. 13/316,402 (now U.S. Pat. No. 8,725,325) (CTI-007). Further, vehicles 808 include vehicle-based switching mechanisms such as those described in co-pending U.S. application Ser. No. 13/323,759 (now U.S. Pat. No. 8,706,328) (CTI-005). Moreover, vehicles 808 preferably include targeted brake rate functionality such as that described in co-pending U.S. application Ser. No. 13/316,398 (now U.S. Pat. No. 8,744,652) (CTI-006).

Generally, collision prevention control system 806 implements embodiments of the collision prevention methodology described herein. Although system 806 is shown as a single entity for ease of illustration, it may include several components, either in the same location or distributed in different locations, for example in or near stations 802. Moreover, collision prevention system 806 further preferably employs a distance monitoring and communication scheme described in more detail in co-pending U.S. application Ser. No. 13/218,434 (now U.S. Pat. No. 8,554,397) (CTI-004). It should be noted that system 800 may further include vehicle control functionality such as that described in co-pending U.S. application Ser. No. 13/323,768 (CTI-008).

A list of attributes for a control system for this invention is preferably implemented by system 806 that achieves the following characteristics:

-   -   Allows operation at headways less than that achievable with the         brick wall criteria which will require         -   Communication based signaling         -   Vehicle borne switches         -   Fail-operational controlled braking     -   Capable of coordinating the movement of a large fleet of         vehicles     -   Fails to an operational state     -   Inexpensive Vehicle-borne equipment     -   Limited to providing longitudinal control     -   Achieves high safety standards     -   Must not allow contact between cars during braking     -   Includes the control of doors     -   Includes the control of vehicle-borne switching hardware     -   Designed for a long service life     -   Cost significantly less to maintain than conventional systems

A sample system design that effectively addresses these desired attributes will be described below.

In embodiments of system 900, several layers of control are included. They are, for the sake of this discussion, the Demand Management Layer 902, the Vehicle Management Layer 904, the Vehicle Control Layer 906, the Vehicle Interface Layer 908, and finally the Physical Layer 910. Physical layer 910 contains the actual physical components that are to be controlled by the control system which are not, strictly speaking, a part of the control function. It is included here in the control system discussion because included here are the physical sensors that implement part of the control function. FIG. 9 shows the hierarchy of these layers and where the system components that comprise each layer might physically reside.

As depicted in this figure, the top two layers, Demand Management 902 and Vehicle Management 904, are typically located at a central control facility 920 and have operational jurisdiction over the whole system. At these level the primary responsibility is the management of the entire guideway network and management of the fleet. No safety critical functions will be performed here.

The next level down, the Control Layer 906, has jurisdiction over relatively small segments of the system, and would likely be housed at the station 922 where structures might exist to house and protect the systems from the elements. The most critical functional responsibility of this layer will be to monitor system performance and to command safe responses to developing unsafe situations before hazards can occur. For this, the control layer 906 would have several tasks: (a) cause vehicles to move as requested by the Vehicle Management Layer 904 (b) monitor the actual physical behavior of all vehicles to confirm the absence of any fault or emergency situation developing within its area of control, and (c) respond safely to any detected unsafe situation occurring locally (within its range) or globally (whole network). Tasks B and C are critical to safety and are what make this layer the only vital layer of the overall system, aspects of which are described in more detail in co-pending U.S. application Ser. No. 13/218,423 (CTI-002).

The Interface Layer 908 in this example is split between the station 922 and vehicle 924. As the name implies, this layer is simply an interface between the station 922 and the vehicle 924 and actually contains no control functionality. The principal reason for limiting the role of this layer is the need to keep the complexity of the equipment on the vehicle 924 as low as possible to address the need to keep the cost of procurement and operation of the vehicle 924 borne equipment low. This puts a heavier burden on the computing systems in the station 922 but with computing systems available today, this should not be unattainable. In addition, and perhaps more critical, is the greater dependence there will be on the communication infrastructure between the components of the Interface Layer 908 in the station 922 and the components on the vehicle 924.

Finally, the Physical layer 910 is composed of the actual elements on the vehicle 924 that are to be controlled. These would be the motors, brakes, and doors, as well as a mechanism to achieve the switching on board the vehicle. Also included are the sensors that allow the monitoring of the behavior of the physical elements of the system.

FIG. 10 illustrates the hierarchical relationship between layers of system 900 according to example embodiments of the invention. To be noted here is the representation that higher levels each have jurisdiction over multiple lower levels thus giving the higher levels broader control jurisdiction.

As mentioned earlier, the higher layers of control, Demand Management 902 and Vehicle Management 904, do not perform safety critical functions. That being said, it is in these layers that the fleet management algorithms will be implemented, and for complex systems, it is this function that will dictate the operational efficiencies of the system. Therefore, careful thought and clever design solutions are preferably used to arrive at an effective system. Also, given that the effectiveness of the system will be dictated by changing ridership patterns, these layers should be designed in ways that allow for easy modifications to adapt to changing use patterns. Limiting the functional scope of these layers to non-safety critical functionality is one way in which this is achieved

The Control Layer 906 is the part of the system that ensures the safety of the system 900. It is therefore the part of the system that is preferably designed with the greatest care, and where necessary, incurs the extra cost of redundancy to ensure safe operation. Since in this sample system representation, the Control Layer 906 is housed in the station 922 from which a large number of vehicles 924 can be controlled, the high investment in the Control Layer 906 components is more easily justified.

FIG. 11 illustrates how the Control 906, Interface 908, and Physical 910 layers are implemented in example embodiments of the invention. It should be noted that this illustration is intended to explain certain aspects of how a system according to the invention might be implemented, so certain details have been omitted for clarity of the invention. In this design, the safety critical control algorithms are all contained within the vital processor in the station 922 and the processing on the vehicle 924 is limited to the task of translation (from digitized commands to analog drive signals). The only information from the station 922 that is critical to safety is the Safety Enable commands, represented here as the Safety Enable signals 1104. These commands are generated in the Vital Station Computer 1102 of the Control Layer 906 where the processing is assured to have integrity. Furthermore, these commands are communicated with multiple layers of encryption (error detection codes) making it very difficult to counterfeit or to misinterpret. Since the proper interpretation of the Safety Enable codes is critical to safety, the method in co-pending U.S. application Ser. No. 13/316,402 (now U.S. Pat. No. 8,725,325) (CTI-007) provides an explanation of how this can be achieved without the use of specially designed and/or redundant processors. (Note: The redundancy shown in FIG. 11 is for system availability, not safety.)

In the station 922, the non safety-critical computing 1106 is performed in a dual redundant computer for fault tolerance and the safety-critical computing is performed in either a triply redundant (with 2 out of 3 voting) or a quad redundant processor (2 times 2 out of 2 voting) for safety and fault tolerance. For motion control, the feedback control algorithms to cause the vehicle to move along a desired position versus time trajectory as directed by the Vehicle Management Layer 904 is contained within a non-vital processor that uses location data from the vehicles and movement trajectories from the Vehicle Management Layer 904 to calculate commands to be sent to the vehicle to cause the tractive effort on the wheels to increase or decrease. The vital processing platform 1102, for motion control, has the relatively simple responsibility to look for conditions that mandate that the “plug be pulled” and manifests the results of this processing as a Safe to Proceed output when the vehicle 924 does not have to be brought to a stop. This algorithm can be designed to consider a moving leading vehicle to be a vehicle that will adhere to a controlled brake rate and allow for movement that violates the brick wall criteria (e.g. instead monitoring the safe stopping distance as illustrated in FIG. 7). For door control, the output from the vital processor 1108 would be a Safe to Open output 1110 that would be generated only when the conditions are such that the opening of doors 1112 is safe. For the vehicle switch 1114 control, the output from the vital processor 1108 would be a Safe to Switch output 1116 that would be generated only when the conditions are safe to change the position of the switch.

The combined output of the station processing is then transmitted via a wireless link to the target vehicle. With proper encryption for error detection included in the message, this link does not have to be vital.

The communication of commands 1118 from the station 922 to the vehicle 924 and reports 1120 from the vehicle 924 to the station 922, can be achieved by a variety of technologies. The least hardware intensive technology would be by direct line of sight radio frequencies (RF) data transceivers 1130. Since the transmission distance from track to vehicle is relatively short, lossy line cables could also be used for RF communication but this would require the installation of cable and repeater amplifiers along the entire length of the trackway. In past implementations, inductive links have been used for the communication path, but again, wire loops and amplifiers would have to be installed throughout the system.

The Physical Layer 910 is depicted in FIG.11 in boxes labeled motors and brakes 1122, doors 1112, switches 1114, and sensors 1124. The latter is a collection of sensors that allow for monitoring the status of the vehicle. The other three are the actual physical devices that are being controlled by the control system.

In the scheme illustrated by FIG. 11, the non-vital processors 1126 on the vehicle 924 interpret commands from the station 922 and generate control signals to the elements being controlled. A separate processor 1108, monitors the received data for the presence of the Safety Enable signals 1104 and only when they are detected, passes STP/STO/STS signals 1110, 1116, 1128 to the controlled elements 1112, 1114 and 1122. The controlled elements in turn are designed to require the presence of the “Safe To” signals to withhold braking, open doors, or move switches. Lacking the enabling signal each individual controlled element will be disabled from taking permissive action (i.e. withhold braking and continue moving, open doors, or move switches) after the elapse of a timeout period.

Although the present invention has been particularly described with reference to the preferred embodiments thereof, it should be readily apparent to those of ordinary skill in the art that changes and modifications in the form and details may be made without departing from the spirit and scope of the invention. It is intended that the appended claims encompass such changes and modifications. 

What is claimed is:
 1. A method of controlling a plurality of driverless vehicles in a fixed guideway system, comprising: determining that there are no fixed obstacles in the system; and maintaining separation between the vehicles using dynamic block control that takes into account a controlled braking rate of one or more of the vehicles.
 2. A method according to claim 1, wherein maintaining separation includes calculating a stopping distance delta between a leading vehicle and a trailing vehicle.
 3. A method according to claim 2, wherein the stopping distance delta includes a worst case stopping distance of the trailing vehicle and a best case stopping distance of the leading vehicle.
 4. A method according to claim 3, further comprising calculating the worst case stopping distance and the best case stopping distance using the controlled braking rate.
 5. A method according to claim 2, wherein maintaining separation includes: continually monitoring the stopping distance delta; and causing certain of the vehicles to stop if the separation between two or more vehicles is less than the stopping distance delta.
 6. A method according to claim 5, further comprising: transmitting a safety enable signal to the vehicles if the monitoring step does not detect any safety issues, wherein the causing step includes withholding transmission of the safety enable signal to the certain vehicles.
 7. A method according to claim 5, wherein continually monitoring the stopping distance delta includes periodically receiving location information from the plurality of vehicles.
 8. A method according to claim 7, wherein communication based train control is used to periodically receive the location information from the plurality of vehicles.
 9. A system for controlling a plurality of driverless vehicles in a fixed guideway system, comprising: an interface in each of the vehicles; and a station controller that communicates with the interface in each of the vehicles and maintains separation between the vehicles using dynamic block control that takes into account a controlled braking rate of one or more of the vehicles.
 10. A system according to claim 9, wherein the station controller maintains separation by calculating a stopping distance delta between a leading vehicle and a trailing vehicle.
 11. A system according to claim 10, wherein the stopping distance delta includes a worst case stopping distance of the trailing vehicle and a best case stopping distance of the leading vehicle.
 12. A system according to claim 11, wherein the station controller calculates the worst case stopping distance and the best case stopping distance using the controlled braking rate.
 13. A system according to claim 10, wherein the station controller maintains separation by continually monitoring the stopping distance delta, and causing certain of the vehicles to stop if the separation between two or more vehicles is less than the stopping distance delta.
 14. A system according to claim 13, further comprising: a transmitter in the station controller that transmits a safety enable signal to the vehicles if the station controller does not detect any safety issues, wherein the station controller withholds transmission of the safety enable signal to the certain vehicles.
 15. A system according to claim 13, further comprising: a receiver in the station controller that periodically receives location information from the plurality of vehicles.
 16. A system according to claim 15, wherein communication based train control is used to periodically receive the location information from the plurality of vehicles. 